1. Field of the Invention
The present invention relates to a method and apparatus for determining the position and topography of an object, and for imaging the surface of the object, and more specifically to a method and apparatus for automatically aligning probe tips to probe pads on a semiconductor wafer.
2. Background Information
After semiconductor circuits have been fabricated on the surface of silicon wafers, they must be tested electrically before the expensive step of dicing and packaging can begin. Test machines that manipulate wafers and arrange for electrical contact to be made with the devices on a wafer are called probers. Electrical signals that test the device are then provided by a separate tester, sometimes referred to as the Automatic Test Equipment or ATE. Probers allow test engineers to precisely contact pads within a device with test probes. The probes come on probe cards that are inserted into the prober machine. The probe card is the interface between the die under test (DUT) and the device tester or ATE. Before contact can be made, an operator must first align the first die to the probes. This is usually done by means of a microscope through which the operator can observe the probes and the device simultaneously. After alignment to the first die is made, the alignment is remembered by the prober, and, given the die size, the prober can then step across the wafer automatically from die to die.
There are several types of probe cards. The most popular are cantilever probe cards in which the probes are needles that are nearly horizontal except at the tip where they are bent sharply downward to contact the device under test. The cantilever probe cards are typically limited to a maximum of approximately 500 probes and testing frequencies under approximately 100 MHz. The cantilever probe typically has a probe tip having a diameter of approximately 1-3 mils (i.e., 1-3 thousandths of an inch), depending upon the size and type of pad to be contacted. Another newer type of probe card is the membrane card that uses a flexible membrane, such as a polyimide membrane, with deposited electrical traces and contact bumps which make contact to the DUT. The electrical traces carry signals to and from the DUT. This type of probe card can have 1200 bumps or more and can be used at testing frequencies well into the GHz region. The membrane probe card contact bump typically has a diameter of approximately 3 mils. A third type of probe card is the IBM COBRA.TM. probe. The COBRA probe uses vertical spring loaded pins to contact the DUT. The COBRA probe card can have over 2000 probes and can be used at testing frequencies well into the GHz regions. Pointed tips having a diameter of approximately 1 mil are used to probe, for example, aluminum or silicon pads, while a flatter probe tip having a diameter of approximately 5 mils is used to probe solder bumps. While the cantilever probe cards have visual access from the top to allow an operator, using a microscope, to align probes to pads, the newer membrane and COBRA probe cards have limited or no access. Thus for theses newer cards, automatic probe-to-pad alignment is necessary. Even for cantilever probe cards, however, automatic probe-to-pad alignment is highly desirable, especially for high pin count applications. In addition to the probe cards described above, other probe cards and tips are well known.
One method for automatic probe to pad alignment (APTPA) is an upward looking camera. This method utilizes a camera to take a picture of a region having the probe tips. The picture produces an image of several probe tips in two dimensions, thus allowing for determination of the position of the probe tips in a plane (e.g., the x-y plane),typically parallel to the plane of the prober stage holding the wafer. The position of the camera relative to the prober's stage is known, so that the x and y coordinates of the probe tips relative to the stage can be determined. Auto-align optics, comprised in a separate imaging system, determine the position of the probe pads on the wafer relative to the stage, so that the position of the probes relative to the pads can be determined, and the stage can be moved by a controller such that the probes are in alignment with the pads in x and y. One drawback with this method is that an additional system is necessary to determine accurately the z coordinate of the probe tips relative to the DUT, so that the pad can be contacted with sufficient force to provide for good electrical contact, without using excessive force that will damage the DUT. A further disadvantage of this method is that the camera system is relatively expensive, greatly adding to the overall cost of the prober.
Another method of APTPA is the use of a dummy wafer in conjunction with auto-align optics. First, the z position of the prober relative to the dummy wafer is determined using a separate system. Next, the dummy wafer, having a soft, markable surface such as an aluminum layer, is probed. Next the marks left by the probe tips on the dummy wafer are examined by the auto-align optics. Thus, knowing the position of the optics relative to the stage, the x-y position of the probe tips relative to the stage can be determined. This method, like the previous method, has the drawback that a separate system to determine the z position of the probe tips is required. Additionally, this method requires the production, inventorying, and tracking of dummy wafers. Additionally, an operator is usually required to load the dummy wafer into the system. Finally, scratches, contamination, and other stray markings can cause the system to fail to determine the positions of the markings.
What is needed is a method and apparatus for determining the position of an object, such as a probe tip, to allow for alignment of the object to another object such as the pads on a device to be probed. The method and apparatus should be able to determine the position in all three dimensions, x, y, and z. Further the method and apparatus should be fully automatic, so that the alignment can be performed automatically, without operator intervention. Finally, the method and apparatus should be relatively inexpensive to implement on a wide variety of systems such as, for example, semiconductor wafer probers.